Prepare the beef: Place the sliced flank steak in a bowl and toss with cornstarch. Let sit for 5 minutes to help it crisp when frying.
Heat the oil: In a large pan or wok, heat the vegetable oil over medium-high heat.
Cook the beef: Fry the beef in batches (don’t overcrowd the pan) for 2-3 minutes per side until crispy and brown. Remove and set aside.
Make the sauce: In the same pan, reduce heat to medium and add garlic and ginger. Cook for 30 seconds. Add soy sauce, brown sugar, water, and red pepper flakes. Stir and simmer for 2-3 minutes until slightly thickened.
Combine: Return the beef to the pan and toss in the sauce until well coated. Cook for another 2 minutes.
Garnish: Sprinkle with green onions and sesame seeds. Serve hot over steamed rice or noodles